The invention pertains to a device for measuring selected dimensions of single-point and multi-point cutters or cutting tools intended for use with a machine tool.
When a computer numerical control is employed to operate a machining center or other machine tool, the control is generally provided with a number of part programs. Execution of a program results in the generation of a series of commands which direct the machine tool to move cutters or cutting tools engaged thereby over specified distances, relative to a part, to automatically machine the part.
In designing a part program, the various distances are typically selected on the assumption that the pertinent dimensions of associated tools, such as tool length or radius, have particular assigned values. However, the actual or true dimensional values of a tool engaged by the machine tool may be significantly different from the assigned, or nominal values. The difference between the nominal and actual values of a tool dimension is referred to as tool offset. It will be readily apparent that the offset values of a tool must be determined and coupled to the numerical control, to enable the control to compensate for the offsets when executing a part program.
U.S. Pat. No. 3,492,467, issued Jan. 27, 1970 for a "Tool Offset Control for Digital Numerically Controlled Point-to-Point Machine Tools" shows an arrangement for measuring the actual length of a tool engaged in a machine tool spindle. The spindle is initially positioned at a spindle zero reference position, and is then advanced toward a tool sensor. As the spindle and engaged tool are moved, a counter generates successive counts which are stored in a register. When the tool reaches the tool sensor, the contents of the register indicate the offset of the engaged tool.
U.S. Pat. No. 4,016,784, issued Apr. 12, 1977 for "Tool Setting Device", shows an arrangement employing a linear variable displacement transducer (LVDT). As taught by such patent, a core of magnetic material is mechanically linked to be moved by a tool, such that displacement of the core corresponds to tool offset. The core moves within a coil form, a primary coil being wound around the center of the form and secondary coils being wound on either side of the primary coil. The difference between the output voltages of the two secondary coils varies relative to the position of the core within the coil form, and may thus be used in determining offset.
U.S. Pat. No. 4,334,178, issued June 8, 1982 for "Method and Apparatus for Correcting Tool Lengths and Tool Radii, Particularly for Numerically Controlled Machines Equipped with Position or Path Measuring System by Means of Nominal-Actual Value Comparison", shows an arrangement in which a tool engaged by a machine tool spindle is moved against a pin to measure tool length or radius. The speed of the tool and pin is decreased to a reference speed, whereupon a moveable member of an inductive transmitter is engaged and moved to a zero position.
Prior art devices of the above type for measuring tool offset may require excessive moving parts, or be characterized by other complexities tending to increase cost and reduce precision of measurement. Also, some of such devices may not be readily adapted to rapid measurement of tool dimensions, such as may be highly desirable when it is necessary to measure the radius of every single tooth of a multi-point tool having in excess of 60 teeth.